Field of the Invention
The present invention is generally directed toward reverse-acting rupture discs having buckling-control structures formed therein. The buckling control structures generally comprise one or more belt regions of greater-thickness material, surrounded by pocket regions of lesser-thickness material, in which regions disc material has been removed, preferably via a laser-ablation process. The belt regions, along with other regions of greater thickness, provide zones of enhanced mechanical properties that assist with burst pressure control and reversal and opening performance of the disc upon initiation of bulged section reversal. The domes of such rapture discs are self-supporting, facilitating relatively simple construction.
Description of the Prior Art
Rupture discs have long been utilized to protect pipelines and process equipment from untoward pressure conditions that if left unchecked could result in equipment damage or loss. Rupture discs have been manufactured in a wide range of sizes and pressure ratings. Even rupture discs of a common size, generally indicated by the diameter of the bulged section in the case of reverse-acting discs, can be required to have a range of operational burst pressures to suit the needs of various particular applications. For example, a one-inch reverse-acting rupture disc may require, in some applications, a burst pressure rating of 75 psi. However, in other applications, a one-inch reverse-acting rupture disc may require a burst pressure rating of 50 psi.
Conventional reverse-acting rupture discs exhibit problems with reliable opening in low energy environments, especially those involving contact of the disc with viscous fluids. The opening sequence of a reverse-acting rupture disc begins with the reversal of the concavity of the bulged section and continues with a rupture or tear of the disc material, starting at one or more point(s), which propagates along a predetermined path, often defined by a line of opening. Progress through these stages requires the continual input of energy. Generally, the initial energy input resulting in reversal of concavity is supplied by the process fluid pressurizing the dome of the disc to the point where it becomes mechanically unstable. As the disc begins to reverse, it may continue to receive energy from the fluid. Meanwhile, elastic energy stored in the compressed material of the disc itself may enhance and accelerate disc reversal. The release and transfer of the stored energy within the metal, in turn, may greatly impact the disc's opening performance. This release of stored energy has been called a “snap-through” effect. In order to induce the internal stresses for this snap-through effect to occur, the bulged section of the disc must possess a shape that allows it to deform while storing, and subsequently releasing, the energy transferred to it from the pressure event, without absorbing and dissipating that energy, especially through excessive plastic deformation. As can be expected, in low pressure events, the energy available to initiate and maintain the full disc opening sequence is quite low, which presents a significant challenge to designing reverse-acting rupture discs that open fully under low pressure conditions.
From a manufacturing perspective, achieving lower and lower burst pressures for a given rupture disc size can be challenging. In some instances, lower nominal burst pressures can be achieved through the use of thinner disc materials or by forming the rupture disc from softer materials, such as nickel and silver. Thinner and softer materials are more susceptible to post-manufacturing damage by relatively benign handling, such as might be encountered in packaging and installation of the discs.
In addition, the use of thinner and softer materials can lead to creation of discs having weaker hinge regions; that is, the region of the disc that secures the petal created upon opening of the bulged section to the remaining bulged or flanged section. Weakened hinge regions increase the possibility of undesirable petal fragmentation. Therefore, at some point using thinner and softer materials to achieve lower burst pressures becomes impractical.
Various alternative methods have been proposed for reducing the pressure at which a disc of a certain size and thickness opens. These methods commonly include weakening the structural integrity of the bulged section of the disc. For example, U.S. Pat. No. 6,494,074 discloses a rupture disc assembly having a depression in the convex surface of the disc. The depression is created by deforming the bulged section of the disc using a tool that is forced against the backed-up convex surface of the rupture disc. The shape, area, and depth of the indentation may be selectively varied in order to achieve the desired loss of structural integrity. However, using this method the range of pressures achievable for a given thickness, with good control and predictability of burst pressure is fairly limited. In seeking to achieve lower pressures, the result is that thinner materials are used, with the handling and mechanical-performance problems noted above.
Very low-pressure discs have also been developed as “composite” structures, built up of two or more members to form an effective whole, combining the advantages of thinner and thicker, and/or softer and stiffer materials to achieve requisite performance. For example, a thin non-self-supporting seal membrane may be placed in conjunction with a thicker support membrane provided with through-cuts to provide for opening. These discs are typically complicated to make and difficult to install, in comparison with discs whose structural component is also the sealing component.